Undersides should be Sky not grey. All colours on US-built airframes for the RAF were "equivalents" to the MAP clolurs but the tonal differences, at least in the specifications, aren't hugely significant. Du Pont 71-021 is often quoted as a US equivalent to Sky but I believe the specific Du Pont shade post-dates the deliveries of P-400s. Other paint manufacturers had their own equivalent shades but it's unlikely a true "grey" was ever one of them.